1. Field of the Invention
The present invention relates to a cadmium sulfide for electrophotography, and more particularly, to a cadmium sulfide produced from starting materials containing hydrochloric acid.
2. Description of the Prior Art
Heretofore, cadmium sulfide for electrophotography has been prepared by passing hydrogen sulfide gas through a solution containing a water soluble cadmium salt such as cadmium sulfate and precipitating cadmium sulfide. The resulting cadmium sulfide particles do not contain any donor and acceptor impurities and is a raw material for preparing photoconductive cadmium sulfide particles for electrophotography. The cadmium sulfide is usually fired together with CuCl.sub.2 as an activator and a halogen compound such as CdCl.sub.2, ZnCl.sub.2 and the like as a flux at elevated temperatures and the impurity is dispersed in the cadmium sulfide particles to activate the particles and thus there is produced a photoconductive cadmium sulfide suitable for electrophotography.
Another prior art process for preparing cadmium sulfide is that disclosed in Japanese Patent Publication No. 14098/1977 where hydrogen sulfide is introduced into a solution containing cadmium ions and impurity element ions to produce cadmium sulfide containing impurities. The impurities can be dispersed in the cadmium particles when fired without adding a flux since impurities are already present therein, and there is obtained a sensitized cadmium sulfide for electrophotography. In particular, according to this process, dispersion of impurities can be effectively conducted upon firing due to the preliminary presence of impurities in cadmium sulfide and unnecessarily large particle size of cadmium sulfide does not form because of the absence of fluxes.
However, cadmium sulfide produced by the prior art processes exhibits disadvantageously a somewhat slow light response. This disadvantage was not a problem, but is now a problem since copying machines are required to be of high speed and a small size. For the purpose of making a photosensitive drum suitable for such copying machines, a seamless photosensitive drum or a small-sized photosensitive drum is adopted. Therefore, taking up one portion of the photosensitive drum, it will be understood that the time interval between formation of a first latent image and that of a second latent image is shortened. When a drum prepared by using a conventional cadmium sulfide is applied to the copying machines, latent images formed in the preceding cycle still remains upon forming latent images in the present cycle and therefore both the preceding pattern and the present pattern overlap and appear simultaneously, so-called "memory phenomenon".
U.S. Pat. No. 4,197,122 discloses a process for preparing cadmium sulfide of high humidity resistance and high sensitivity which comprises precipitating cadmium sulfide in a reaction solution in the presence of hydrochloric acid of a low concentration.
According to the present invention, the process of said U.S. Patent is utilized to provide a cadmium sulfide for electrophotography free from memory phenomenon.